Electrode system for use with an implantable cardiac patient monitor

ABSTRACT

An implantable cardiac monitor is arranged for detecting both arrhythmias and ischemia of the human heart. The monitor includes subcutaneous electrodes for establishing electrical contact with the heart and a sense amplifier coupled to each electrode for generating an electrocardiogram of a heart beat sensed at each of the electrodes. The electrocardiograms are digitized and the digital samples thereof are stored in a memory. A microprocessor processes the digital samples of the electrocardiograms and generates characterizing data indicative of the physiology of the heart. The cardiac monitor includes telemetry to permit the cardiac data to be interrogated externally of the patient for obtaining the generated cardiac data indicative of arrhythmic and ischemic episodes.

This is a division of application Ser. No. 07/820,580, filed Jan. 14,1992, now U.S. Pat. No. 5,313,953.

BACKGROUND OF THE INVENTION

The present invention generally relates to a cardiac monitor. Thepresent invention more particularly relates to a fully implantablecardiac monitor for monitoring the physiology of the heart and which isexternally programmable for detecting either arrhythmias of the heart orischemia or both. The implantable cardiac monitor generates dataindicative of these conditions and stores the data in memory for laterretrieval externally of the patient through telemetry. The presentinvention is further directed to electrode systems for use with theimplantable cardiac monitor for sensing heart activity.

Coronary artery circulation normally supplies sufficient blood flow tothe heart to meet the demands of the heart muscle (myocardium) as itlabors under a widely varying workload. An imbalance that arises betweenthis supply and demand usually precipitates angina pectoris (pain). Whenthe imbalance becomes excessive, myocardial infarction results.Myocardial infarction is necrosis or death of cardiac tissue resultingfrom the lack of blood flow to the heart. For example, the narrowing ofa major coronary artery by more than fifty percent (50%) impairsnutrient blood flow under conditions of myocardial demand.

By far the most common underlying pathologic process that gives rise tothe narrowing of a major coronary artery is atherosclerosis. In mostpatients suffering from atherosclerosis, plaque develops in the proximalsegments of the coronary arteries. In other patients, this condition maybe diffuse and may occur in both proximal and distal vessels.

Increases in oxygen consumption cause ischemia if coronary artery bloodflow cannot rise to meet a higher demand. The clinical manifestations ofischemia are angina, myocardial infarction, congestive heart failure,and electrical instability (arrhythmia). The last mentioned symptom isassumed to account for most of the sudden cardiac death syndromepatients.

Silent ischemia (ischemia without angina) is common and may result in amyocardial infarction without warning. It has been reported thattwenty-five percent (25%) of patients hospitalized with a myocardialinfarction have no pain and over fifty percent (50%) of ischemicepisodes occur without associated pain.

In treating ischemia, the primary goal of medical therapy is to reduceoxygen consumption and increase blood supply by reducing vascular tone(improving collateral flow) preventing thrombosis and opening orbypassing the blockage in the artery or arteries affected. If a clot iscausing the blockage, a thrombolytic drug may be used to open theoccluded artery. The most direct way to increase blood supply is torevascularize by coronary artery bypass surgery or angioplasty.

Cardiac electrical instability (arrhythmia) may occur during ischemicevents and is also a common condition after a myocardial infarction.Since cardiac arrhythmias such as ventricular tachycardia can degenerateinto ventricular fibrillation which is life threatening, thesearrhythmias are of great concern to the physician or cardiologist. Tocontrol such arrhythmias, the cardiologist may choose to treat thepatient with antiarrhythmic drugs.

The testing of the effectiveness of such drugs in reducing the numberand severity of arrhythmias is very difficult. This is due to the factthat arrhythmias occur at any and all times. In attempting to test theeffectiveness of such drugs, patients are often required to wear anexternal monitor for periods of twenty-four (24) or forty-eight (48)hours that record all cardiac signals during these periods. Also, thephysician may submit a patient to extensive electrophysiologic testingwhich is often performed in a hospital. The physician then uses theresults of such testing to assist in determining the course of such drugtherapy.

Myocardial infarctions often leave patients with a permanentarrhythmogenic condition even after coronary artery bypass surgery orangioplasty. Such patients are in need of close monitoring for botharrhythmic events as well as further deterioration of the patency of thecardiac vessels.

After revascularization to increase blood supply to the myocardium, thecardiologist must continually submit such patients to diagnostic teststo determine if the revascularization procedure has remained effective.In angioplasty patients, studies have indicated that twenty-five percent(25%) of those patients will experience restenosis within a period ofsix (6) months. In those patients having coronary artery bypass surgery,restenosis may occur anywhere from a few hours to several years from thetime of such surgery. Studies have indicated that after approximatelyfive (5) years, patients having coronary artery bypass surgery should bemonitored closely.

To diagnose and measure ischemic events suffered by a patient, thecardiologist has several tools from which to choose. Such tools includetwelve-lead electrocardiograms, exercise stress electrocardiograms,Holter monitoring, radioisotope imaging, coronary angiography,myocardial biopsy, and blood serum enzyme tests. Of these, thetwelve-lead electrocardiogram (ECG) is generally the first procedure tobe utilized to detect a myocardial infarction. An exercise stresselectrocardiogram is generally the first test to be utilized fordetecting ischemia since resting twelve-lead electrocardiograms oftenmiss the symptoms of ischemia. Unfortunately, none of the foregoingprocedures provide an ongoing and continuous evaluation of a patient'scondition and are therefore only partially successful at providing thecardiologist with the information that the cardiologist requires indetermining the proper corrective course of action.

There is therefore a need in the art for an implantable cardiac patientmonitor capable of providing twenty-four (24) hour a day monitoring ofpatients for either the sudden onset of restenosis or a new occlusion ora serious arrhythmic event. The present invention provides such animplantable cardiac patient monitor. By virtue of the present invention,long-term trends in ischemia, heart rates and arrhythmias may bemonitored and recorded. Also, by virtue of the present invention,high-risk patients can be instructed to seek aid immediately to avoidpermanent cardiac tissue damage due to a thrombus. In addition, byvirtue of the present invention, the cardiologist can use the ischemiatrend data to guide further therapy to match changing conditions of apatient whether the patient is improving or deteriorating. Arrhythmiascommon to myocardial infarction patients may also be monitored and theseconditions may also be trended as well. Such information can beespecially useful to the cardiologist in adjusting antiarrhythmic drugtherapy to maximize such therapy and minimize side effects. Hence, theimplantable cardiac patient monitor of the present invention is capableof providing the cardiologist with ischemic and heart rhythm informationnot previously available in the prior art which will enable a physicianto eliminate or delay certain diagnostic tests and enable the physicianto maximize drug therapy.

SUMMARY OF THE INVENTION

The present invention provides a cardiac monitor for monitoring thephysiology of a human heart. The monitor is fully implantable beneaththe skin of a patient and includes electrode means for establishingelectrical contact with the heart, sensing means coupled to theelectrode means for generating an electrocardiogram of each heart beatof the heart and processing means responsive to the electrocardiogramscorresponding to natural heart beats for detecting arrhythmias of theheart and generating arrhythmia data characterizing the arrhythmias. Thecardiac monitor further includes memory means coupled to the processingmeans for storing the arrhythmia data and telemetry means fortransmitting the arrhythmia data to a nonimplanted external receiver.

The processing means is further responsive to the electrocardiograms fordetecting ischemia of the heart and is programmable by an externalprogrammer for detecting arrhythmias of the heart, for ischemia of theheart, or for detecting both arrhythmias and ischemia of the heart.

The present invention further provides a cardiac monitor for monitoringthe physiology of a human heart wherein the monitor is fully implantablebeneath the skin of a patient. The cardiac monitor includes electrodemeans for establishing electrical contact with the heart, sensing meanscoupled to the electrode means for generating an electrocardiogram ofeach heart beat of the heart and data generating means coupled to thesensing means for generating electrocardiogram data for each generatedelectrocardiogram. The cardiac monitor further includes processing meansresponsive to the electrocardiograms corresponding to natural heartbeats for processing the electrocardiogram data to generatecharacterizing data indicative of the physiology of the heart and memorymeans coupled to the data generating means and to the processing meansfor storing the electrocardiogram data and the characterizing data. Theprocessing means obtains the electrocardiogram data from the memorymeans and processes the electrocardiogram data at times in between theheart beats.

The present invention further provides a cardiac monitor for monitoringthe physiology of a human heart wherein the monitor is fully implantablebeneath the skin of a patient and includes electrode means includingfirst and second electrodes for establishing electrical contact with theheart to detect heart beats of the heart, sensing means coupled to thefirst and second electrodes for generating first and second respectiveelectrocardiograms of each heart beat of the heart and processing meansresponsive to the first and second electrocardiograms for detectingarrhythmias of the heart and generating arrhythmia data characterizingthe arrhythmias. The cardiac monitor further includes memory meanscoupled to the processing means for storing the arrhythmia data andtelemetry means for transmitting the arrhythmia data to a nonimplantedexternal receiver.

The present invention still further provides an electrode system for usewith a fully implantable cardiac monitor of the type includingelectrical circuitry for monitoring the physiology of the human heartand having an enclosure for containing the electrical circuitry whereinthe enclosure includes an upper perimeter. The electrode system includesan electrically insulating header assembly sealingly engaged with theupper perimeter of the enclosure, first and second flexible insulativeconduits extending from the header, and first and second electrode meanscarried by each of the first and second conduits respectively. The firstand second electrode means each include at least one electricallyconductive electrode. The electrode system further includes conductormeans extending through the first and second conduits and into theheader for coupling the electrodes of the first and second electrodemeans to the electrical circuitry of the monitor. The conduits and theelectrode means are implantable beneath the skin of a patient to disposethe electrodes in non-touching proximity to the heart for establishingelectrical contact between the electrodes and the heart.

The present invention still further provides a cardiac monitor formonitoring the physiology of the human heart wherein the monitor isfully implantable beneath the skin of a patient. The cardiac monitorincludes a hermetically sealed enclosure defining a cavity having anopened perimeter, and a header sealingly engaging the opened perimeter.The cardiac monitor further includes first and second electricalconductors covering first and second discrete portions of the enclosurefor forming first and second sensing electrodes respectively for sensingactivity of the heart, a third electrical conductor covering a thirddiscrete portion of the enclosure for forming a reference electrode andcircuit means within the enclosure and coupled to the electrodes formonitoring the activity of the heart sensed by the sensing electrodes.

The present invention further provides a cardiac monitor for monitoringthe physiology of the human heart wherein the monitor is fullyimplantable beneath the skin of a patient and includes a hermeticallysealed enclosure including a bottom perimeter, and an electricallyinsulating header sealingly engaging the bottom perimeter, at least oneelectrode for sensing activity of the heart, circuit means within theenclosure and coupled to the at least one electrode for monitoring theactivity of the heart sensed by the at least one electrode and forgenerating data indicative of the monitored activity of the heart andtelemetry means disposed within the header for transmitting the data toa nonimplanted external receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The invention,together with further objects and advantages thereof, may best beunderstood by making reference to the following description taken inconjunction with the accompanying drawings, in the several figures ofwhich like reference numerals identify identical elements, and wherein:

FIG. 1 is a schematic front plan view of the human abdomen and chestillustrating a preferred implantation site of an implantable cardiacmonitor embodying the present invention;

FIG. 2 is a graphic representation of a typical or normal ECG waveformshowing the conventional nomenclature for the various portions thereof;

FIG. 3 is a detailed schematic block diagram of the internal circuitryof an implantable cardiac monitor embodying the present invention;

FIG. 3A is a more detailed block diagram of the random access memory ofFIG. 3 illustrating the parameters stored within the random accessmemory for arrhythmia analysis and ischemia analysis in accordance withthe preferred embodiment of the present invention;

FIG. 4 is a graphic representation of a template which is generated andutilized by the implantable cardiac monitor of FIG. 3 for discriminatingbetween normal and abnormal heart beats in accordance with the presentinvention;

FIG. 5 is another graphic representation of a typical, normal ECGwaveform showing fiducial reference points determined by the implantablecardiac monitor of FIG. 3 in accordance with the present invention foranalyzing arrhythmias of the heart;

FIG. 6 is another graphic representation of a typical or normal ECGwaveform showing fiducial reference points determined and used by theimplantable cardiac monitor of FIG. 3 in accordance with the presentinvention for analyzing ischemia;

FIG. 7 is an overall flow diagram illustrating the manner in which theimplantable cardiac monitor of FIG. 3 may be implemented for monitoringthe physiology of the human heart;

FIGS. 8A and 8B when taken together are a flow diagram illustrating themanner in which the implantable cardiac monitor of FIG. 3 may beimplemented for analyzing a heart beat in accordance with the presentinvention;

FIG. 9 is an overall flow diagram illustrating the manner in which theimplantable cardiac monitor of FIG. 3 may be implemented for performingthe channel analysis of the flow diagram of FIGS. 8A and 8B inaccordance with the present invention;

FIG. 10 is a flow diagram illustrating the manner in which theimplantable cardiac monitor of FIG. 3 may be implemented for determiningthe fiducial points illustrated in FIG. 5 in accordance with the presentinvention;

FIG. 11 is a flow diagram illustrating the manner in which theimplantable cardiac monitor of FIG. 3 may be implemented for classifyingheart beats in accordance with the present invention;

FIG. 12 is a flow diagram illustrating the manner in which theimplantable cardiac monitor of FIG. 3 may be implemented for matching adetected ECG to a stored template as illustrated in FIG. 4 in accordancewith the present invention;

FIGS. 13A and 13B when taken together form a flow diagram illustratingthe manner in which the implantable cardiac monitor of FIG. 3 may beimplemented for classifying heart beat rhythms in accordance with thepresent invention;

FIG. 14 is an overall flow diagram illustrating the manner in which theimplantable cardiac monitor of FIG. 3 may be implemented for adaptingthe ECG template and performing ischemia analysis at spaced apart timeintervals;

FIG. 15 is a flow diagram illustrating the manner in which theimplantable cardiac monitor of FIG. 3 may be implemented for revisingthe ECG template at spaced apart time intervals and determining ischemiaanalysis fiducial points in accordance with the present invention;

FIG. 16 is a flow diagram illustrating the manner in which theimplantable cardiac monitor of FIG. 3 may be implemented for performingischemia analysis in accordance with the present invention;

FIG. 17 is a front plan view of an implantable cardiac monitor includingan electrode system employing catheter electrodes for sensing heartactivity and configured in accordance with one preferred embodiment ofthe present invention;

FIG. 18 is a front plan view of an implantable cardiac monitor includinganother electrode system employing catheter electrodes for sensing heartactivity and configured in accordance with another preferred embodimentof the present invention;

FIG. 19 is a front plan view of an implantable cardiac monitor includinga further electrode system employing catheter electrodes for sensingheart activity and configured in accordance with a further preferredembodiment of the present invention;

FIG. 20 is a front plan view of an implantable cardiac monitor includingan electrode system employing strip electrodes for sensing heartactivity and configured in accordance with another preferred embodimentof the present invention;

FIG. 21 is a front plan view of an implantable cardiac monitor includinga further electrode system employing strip electrodes for sensing heartactivity and configured in accordance with a further preferredembodiment of the present invention;

FIG. 22 is a front plant view of an implantable cardiac monitorincluding a further electrode system employing strip electrodes forsensing heart activity and configured in accordance with a still furtherpreferred embodiment of the present invention to provide strain relieffor the strip electrodes;

FIG. 23 is a front plan view of an implantable cardiac monitor includinga still further electrode system employing strip electrodes for sensingheart activity and configured in accordance with a still furtherpreferred embodiment of the present invention for providing strainrelief for the strip electrodes;

FIG. 24 is a top plan view of one of the strip electrodes in conjunctionwith a positioning tool which may be utilized in accordance with thepresent invention for positioning the strip electrode during theimplantation thereof;

FIG. 25 is a side plan view of the strip electrode of FIG. 24;

FIG. 26 is a front plan view of an implantable cardiac monitor embodyingthe present invention which employs a leadless electrode system inaccordance with a further embodiment of the present invention; and

FIG. 27 is a front plan view of an implantable cardiac monitorillustrating a preferred location for telemetry means for efficientlytransmitting data characterizing the physiology of the heart from theimplantable cardiac monitor to an external receiver.

DETAILED DESCRIPTION

Referring now to FIG. 1, it is a schematic front plan view of the humanabdomen and chest illustrating a preferred implantation site of animplantable cardiac monitor 30 embodying the present invention. Theimplantable cardiac monitor 30 generally includes an enclosure 32 andfirst and second electrode means 34 and 36. The enclosure 32, as will bedescribed hereinafter, includes electronic circuitry for monitoringheart activity and generating data indicative of the physiology of theheart. The electrode means 34 and 36 are coupled to the electroniccircuitry within the enclosure 32 by conductor means extending throughconduit means in a manner to also be described in greater detailhereinafter.

The enclosure 32 is preferably implanted beneath the skin in the leftabdominal area below the diaphragm in the rib cage. The electrode means34 and 36 preferably comprise subcutaneous electrodes which are alsoimplanted beneath the skin for establishing electrical contact with theheart in non-touching relation thereto. Illustrated in FIG. 1 are sixstandard locations for external exploring electrodes used for routineclinical electrocardiography designated V₁ through V₆. The electrodemeans 34 and 36, as illustrated, are implanted in the precordial area inclose proximity to the V₂ through V₆ locations. As will be seenhereinafter, each of the electrode means 34 and 36 may comprise stripelectrodes including one or more discrete conductive electrodes orcatheter electrodes including one or more conductive ring-shapedelectrodes.

Referring now to FIG. 2, it provides a graphic representation of atypical or normal electrocardiogram (ECG) waveform showing theconventional nomenclature for the various portions thereof. Thebeginning of a heart beat is initiated by a P wave which is normally asmall positive wave. Following the P wave there is an ECG waveformportion which is substantially constant in amplitude. This substantiallyconstant portion will have a time duration on the order of, for example,120 milliseconds and may be utilized for establishing a baseline fordetecting ischemia.

The QRS complex of the ECG then normally occurs after the substantiallyconstant portion with a Q wave which is normally a small negativedeflection which is then immediately succeeded by the R wave which is arapid positive deflection. The R wave generally has an amplitude greaterthan any other waves of the ECG signal and will have a spiked shape ofrelatively short duration with a sharp rise, a peak amplitude, and asharp decline. The R wave may have a duration on the order of 40milliseconds. However, as described hereinafter, the cardiac monitor 30distinguishes between normal heart beats of the type illustrated in FIG.1, for example, and abnormal heart beats which are referred to herein asventricular beats which are ectopic beats originating in a ventricle ofthe heart and which is generally characterized by an R wave having aduration which is greater than the duration of the normal R wavemorphology of the patient being monitored.

Following the R wave, the QRS complex is completed with an S wave. The Swave may be generally characterized by a small positive inflection inthe ECG signal.

Following the S wave is the T wave which is separated from the S wave bythe ST segment. The amplitude of the ST segment, in a healthy heart, isgenerally approximately equal to the baseline following the P wave andpreceding the Q wave. As will be seen hereinafter, the cardiac monitor30 detects ischemia when the amplitude of the ST segment deviates fromthe baseline following the P wave by an amount greater than apredetermined amount. The T wave is relatively long in duration of, forexample, on the order of 150 milliseconds. Following the T wave, whichconcludes the heart beat, is a substantially constant amplitude untilthe next P wave occurs.

As will be seen hereinafter, each electrode of the cardiac monitor 30 iscoupled to a respective input amplifier. Each input amplifier generatesan ECG signal for each heart beat which is digitized by an analog todigital converter and stored in a memory through a direct memory access.Following each heart beat, a microprocessor of the cardiac monitorprocesses the stored data and generates data indicative of thephysiology of the heart being monitored. The microprocessor processesthe data and generates the characterizing data after the digital samplesof the ECG signals are stored for each heart beat during the timefollowing the T wave of one heart beat and before the P wave of the nextheart beat, In processing the stored data, the microprocessordistinguishes between normal heart beats (normal sinus heart beats) andabnormal heart beats (ventricular beats) and logs in memory criticalevents after beat classification and classifies heart beat rhythms in amanner to be fully described hereinafter.

Referring now to FIG. 3, it illustrates in schematic block diagram form,the internal circuitry of the implantable cardiac monitor 30 of FIG. 1which is contained within the enclosure 32. The cardiac monitorcircuitry 40 generally includes a plurality of inputs 42, 44, 46, 48,50, and 52 which are arranged to be coupled to the electrodes of theelectrode means 34 and 36 illustrated in FIG. 1. As will be noted, sixsuch inputs are provided for accommodating electrode means having atotal of up to six electrodes. As will be seen hereinafter, inaccordance with this preferred embodiment, up to four of the heartactivity signals received at inputs 42, 44, 46, 48, 50, and 52 may beutilized for monitoring the physiology of the heart. The particularinputs to be utilized in monitoring the physiology of the heart areexternally programmable to allow the cardiologist flexibility inselecting those inputs which provide the best heart activity signals.

The circuitry 40 further includes defibrillation protection circuitry54, a sensing means 56 comprising a plurality of input amplifiers witheach input amplifier corresponding to a respective given one of theinputs. To that end, input amplifier 62 corresponds to input 42, inputamplifier 64 corresponds to input 44, and input amplifier 72 correspondsto input 52. The input amplifiers corresponding to inputs 46, 48, and 50are not illustrated so as to not unduly complicate the figure.

The circuitry 40 further generally includes a multiplexer 74, a datagenerating means 76 including a sample and hold 78 and an analog todigital converter 80, a memory means 82 including a random access memory84 and a read only memory 86, and a direct memory access 88. Thecircuitry 40 further includes a processing means 90 including amicroprocessor 92, a pacemaker detector 94, an R wave detector 96, andan interrupt request 98. The circuitry 40 still further generallyincludes a telemetry input means 100 including a receiver 102 and aprogram access decoder 104, a telemetry output 106, a crystal oscillator108, and an RC oscillator 110. Lastly, the circuitry generally includesa battery monitor 112 and a patient alarm 114.

The defibrillation protection circuitry 54 protects the circuitry 40from defibrillating energy which may be applied to the heart by aventricular defibrillator. Such circuitry may include zener diodes in amanner well known in the art.

The inputs 42, 44, 46, 48, 50, and 52 are coupled to the inputs of theinput amplifiers 62, 64, and 72 through the defibrillation protectioncircuitry 54. Each of the input amplifiers generates anelectrocardiogram representing the heart beats of the heart detected byits corresponding electrode. The outputs of the input amplifiers 62, 64,and 72 are coupled to the multiplexer 74 which, responsive to externalprogramming, selects up to four outputs of the input amplifiers to beutilized for monitoring the physiology of the heart. As a result, theoutput of the multiplexer 74 includes four channels which are coupled tothe sample and hold 78. As illustrated in the Figure, theelectrocardiograms provided by the first and second channels of themultiplexer are used for detecting R waves and are thus coupled to the Rwave detector 96. In addition, the first and second channels of themultiplexer 74 are also coupled to the pacemaker detector 94 fordetecting stimuli applied to the heart by a pacemaker. Such pacemakerdetection is provided so that only those electrocardiogramscorresponding to spontaneous or natural heart beats of the heart areutilized by the processing means 90 for processing the electrocardiogramdata and generating data characterizing the physiology of the heart. Tothat end, the pacemaker detector 94 is coupled to the microprocessor 92to cause the microprocessor to disregard electrocardiograms whichcorrespond to heart activity resulting from a pacemaker stimulus.

The first and second channels of multiplexer 74 along with the third andfourth channels of multiplexer 74 are coupled to the sample and hold 78.The sample and hold 78 is coupled to the analog to digital converter 80which converts the analog electrocardiogram signals being held by thesample and hold 78 to digital samples one at a time in succession. Tothat end, the analog to digital converter 80 is coupled to the crystaloscillator 108 which provides clocking signals at a rate of, forexample, 32 kilohertz. The crystal oscillator 108 continuously providesthe clocking signals so that the sample and hold 78 and analog todigital converter 80 continuously generate digitized electrocardiogramdata. The digital samples provided by the analog to digital converter 80are preferably multiple-bit digital samples containing, for example,nine bits. The digital samples of the electrocardiograms are provided tothe direct memory access 80 which continuously stores theelectrocardiogram digital samples in the random access memory 84.

In addition to storing the digital samples of the electrocardiograms ofeach of the four utilized channels, the random access memory 84 alsostores operating instructions for microprocessor 92 which define theexecutions to be performed by the microprocessor 92 for processing theelectrocardiogram digital samples for in turn generating characterizingdata of the physiology of the heart. As will be seen hereinafter, themicroprocessor 92 responsive to the operating instructions provided byrandom access memory 84 and the electrocardiogram digital samples isarranged for monitoring arrhythmias of the heart, ischemia, or botharrhythmias and ischemia depending upon the manner in which the cardiacmonitor is externally programmed. As will be seen in FIG. 3A, the randomaccess memory 84 also includes storage locations which are utilized forbuffering data to temporarily store such data and storage locations forstoring data generated by the microprocessor 92 which is to be morepermanently stored and made available to the cardiologist upon externalinterrogation for the transmission of such data by the telemetry output106 to an external receiver.

The read only memory 86, in a manner well known in the microprocessorart, stores basic operating system instructions for the microprocessor92. Such basic system operating instructions may include instructionswhich permit the microprocessor 92 to perform the input programming andthe output telemetry functions for transmitting data to and from anexternal receiver, to permit the microprocessor to perform resetexecutions, and to permit the microprocessor to perform self-checkoperations, for example.

As previously mentioned, the microprocessor 92 processes the storedelectrocardiogram digital samples and generates characterizing dataindicative of the physiology of the heart. Because the cardiac monitorcircuitry 40 is implantable, it is preferably powered by a depletablepower source such as a battery. To conserve on battery power, themicroprocessor 92 only processes data at selected times, as for example,between heart beats. When the microprocessor 92 processes data, the RCoscillator 110 provides the microprocessor 92 with clock pulses tocontrol the execution rate of the microprocessor 92. When themicroprocessor is not processing data, the RC oscillator 110 isselectively turned off.

To "wake-up" the microprocessor 92, to permit the microprocessor 92 toprocess data, the R wave detector 96 detects an R wave from the firstchannel, the second channel, or both the first and second channels.After a predetermined time duration following the detection of an Rwave, the R wave detector 96 provides a trigger signal to the interruptrequest 98. The interrupt request 98 services the trigger signal tocause the microprocessor 92 to start the RC oscillator 110 and commenceprocessing data. The predetermined time period or delay in providing thetrigger by the R wave detector 96 may be, for example, a period of 300milliseconds, for example, following the R wave detection to cause themicroprocessor 92 to commence processing data prior to the next heartbeat. As a result, in accordance with this preferred embodiment, therandom access memory 84 need only store the electrocardiogram data for asingle electrocardiogram for each of the four channels. After theprocessing of the electrocardiogram data, the new electrocardiogramdigital samples for the next heart beat may be utilized to write overthe electrocardiogram data stored during the previous heart beat.However, as will be seen hereinafter, at times digital samples ofselected electrocardiograms are to be more permanently stored for laterretrieval by the cardiologist. In such a case, the digital samples ofthe electrocardiograms to be more permanently stored will be moved bythe microprocessor 92 to a more permanent storage location within therandom access memory 84 prior to the occurrence of the next heart beat.The more permanently stored electrocardiograms may be theelectrocardiograms occurring at the onset and termination of variousarrhythmic episodes such as ventricular tachycardia or ischemicepisodes.

The patient alarm 114 is provided to alert the patient to a low batterycondition, a serious arrhythmic event, or a serious ischemic event andto notify the patient that the patient should call the cardiologist. Thepatient alarm 114 may take the form of a piezo electric buzzer forexample or a low energy stimulus which may be felt by the patient butnot of sufficient energy to stimulate the heart. Such alarms may also becoded to permit the patient to inform the cardiologist as to the type ofevent which prompted the alarm.

For programming the modalities of the cardiac monitor, the receiver 102receives a signal generated externally. The programming signal may becoded in a known manner to define the modality of the cardiac monitorand to set certain operating conditions such as heart rate thresholdlevels or ST segment deviation threshold levels for detecting ischemia.The programming signals received by receiver 102 are decoded by theprogram access decoder 104 and conveyed to the interrupt request 98. Theinterrupt request 98 services the decoded programming signals andprovides the same to the microprocessor 92. The microprocessor 92 thenstores the programing operating conditions in the random access memory84 and is also conditioned for fetching only those program instructionsfrom the random access memory 84 for executing the programmedmodalities. For example, the random access memory 84 may store a firstset of operating instructions to cause the microprocessor to detectarrhythmias and a second set of operating instructions to cause themicroprocessor to detect ischemia. If the microprocessor 92 is onlyprogrammed for detecting and monitoring arrhythmias, it will only accessthe first set of operating instructions. If the microprocessor 92 isonly programmed for detecting and monitoring ischemia, it will onlyaccess the second set of operating instructions. If the microprocessoris programmed for detecting and monitoring both arrhythmias andischemia, it will access both the first and second sets of operatinginstructions.

To transmit the characterizing data generated by the microprocessor 92to an external receiver, the telemetry output 106 may include a radiofrequency transmitter of the type well known in the art which transmitsa radio frequency carrier which is pulse code modulated. The radiofrequency signal generated by the telemetry output 106 is radiated froman antenna such as antenna coil 116. A preferred location of thetelemetry antenna coil 116 for efficiently conveying the characterizingdata to an external receiver will be described subsequently.

Lastly, the battery monitor 112 monitors the voltage of the batterywhich powers the cardiac monitor. When the battery voltage decreases toa threshold limit, the battery monitor 112 will provide a signal to themicroprocessor 92 indicating that battery power will soon be depleted.In response to such a signal, the microprocessor 92 may cause thepatient alarm 114 to provide a suitable alarm to the patient to promptthe patient to notify the cardiologist of the low battery condition. Inaddition, the microprocessor 92 may store the battery condition in therandom access memory 84 and time stamp the low battery condition so thecardiologist upon retrieving the characterizing data from the randomaccess memory will be informed as to the time in which the batterymonitor 112 first detected the low battery condition.

Referring now to FIG. 3A, it illustrates in greater detail, the randomaccess memory 84 illustrated in FIG. 3. In addition to storing theoperating instructions for the microprocessor, the random access memory84 includes reserved storage locations for storing various types ofarrhythmia and ischemia data. The random access memory 84 may be dividedinto an arrhythmia portion 85 and an ischemia portion 121. Thearrhythmia portion includes a ventricular beat string counter 87 formaintaining the number of consecutively occurring ventricular beats anda bigeminy string counter 89 for maintaining a count of the number ofconsecutive cycles of a bigeminy rhythm. The arrhythmia portion 85further includes a plurality of event counters including a nonsustainedventricular tachycardia event counter 91, a bigeminy event counter 93and a premature ventricular contraction (PVC) event counter 95. Theevent counters further include a ventricular ectopic beat event counter97, a premature atrial contraction (PAC) event counter 99, and anirregular hear rhythm event counter 101. All of the event counters areutilized for maintaining trend data with respect to the arrhythmiasdetected and analyzed by the implantable cardiac monitor.

The arrhythmia portion 85 further includes a total ventricular beatcounter 103. This counter maintains a count of the total ventricularbeats which are detected over a given time period, such as, each hour.The arrhythmia portion 85 further includes an arrhythmia log forrecording and time stamping sustained arrhythmia episodes. Thearrhythmia log includes ventricular tachycardia event data 105, bigeminyevent data 107, and irregular heart rhythm event data 109. When data isstored in the arrhythmia log, it is made available for telemetry to thecardiologist for retrieval. The data stored in the arrhythmia log istime stamped so that the cardiologist will be advised as to the date andtime in which the sustained arrhythmic episodes occurred. Lastly, thearrhythmia portion 85 includes a plurality of ECG strip stores includinga ventricular tachycardia strip store 111 and an irregular heart rhythmstrip store 113 which would include heart beat variability or pauses.When these sustained rhythms are recorded in the arrhythmia log, thecardiac monitor will also move ECG data which is stored in the randomaccess memory 84 in temporary stores to the more permanent ECG stripstores. The ECG data stored in the ECG strip stores may correspond tothe ECG data generated at the onset of a ventricular tachycardia orirregular heart rhythm and the ECG data generated during the terminationof the ventricular tachycardia.

The ischemia portion 121 includes a counter 123 which is incrementedeach time ischemia is detected. The counter 123 therefore forms anischemia timer with which the duration of an ischemia episode may bemaintained. The ischemia portion further includes episode log 125wherein ischemic episode data is maintained. Such data may include STsegment level data for the onset of ischemia, for the peak of theischemia, and the termination of the ischemia, for example. The ischemiaportion 121 also includes an event counter 127 which keeps track of thenumber of ischemic events which have occurred. Trend data is stored inan ischemia trend data store portion 129. This trend data may be thenumber of ischemic episodes occurring during each hour for example. Theischemia episode log 125 may be utilized for storing ST level trend datafor retrieval by the cardiologist. Lastly, the ischemia portion 121includes an ECG strip store 133 for storing the ECG data generated atthe onset of a sustained ischemic episode, for storing the ECG datagenerated during the peak of the ischemic episode, and the ECG datagenerated at the termination of the sustained ischemic episode.

One important function performed by the processing means 90 inprocessing the electrocardiogram data samples of each natural heart beatis the discrimination between a normal sinus heart beat and an abnormalheart beat herein referred to as a ventricular beat. More specifically,the abnormal heart beat is an ectopic ventricular heart beat wherein theheart beat originates in the ventricles rather than at the sinus nodewhere a normal heart beat originates. Such a ventricular heart beat ischaracterized by an R wave having a longer duration than the R wave of anormal sinus heart beat. In order to facilitate the discriminationbetween a normal sinus heart beat and a ventricular beat, themicroprocessor 92 of the processing means 90 establishes a templatecorresponding to the electrocardiogram of a normal sinus heart beat ofthe patient. The microprocessor 92 generates such a template upon theinitialization of the cardiac monitor and, in accordance with thepresent invention, revises the template at spaced apart time intervalsto account for changes in the normal morphology of the patient's heartover time. Such revisions to the template may be made at periodicintervals of, for example, 15 seconds, or alternatively may be madeafter a predetermined number of heart beats have occurred, such as, forexample, 15 heart beats. In generating the template, the microprocessoraverages a first predetermined number of data samples for each datapoint for a corresponding number of electrocardiograms and ascribes toeach data point a maximum limit and a minimum limit. Such a template 120is illustrated in FIG. 4. The maximum limits are denoted by the dashedline 122 and the minimum limits are denoted by the dashed line 124.While the template illustrated in FIG. 4 expands the entireelectrocardiogram, in accordance with the present invention, thetemplate 120 may span only the QRS portion of the electrocardiogram.

To determine if a heart beat is a normal sinus heart beat or an abnormalheart beat, the stored data samples of the electrocardiogram beingprocessed are aligned with the template. Then, the deviation between thedata samples of the electrocardiogram being processed and the templatefor each data point are summed in a running total until each data sampleof the electrocardiogram being processed has been compared to thetemplate. Thereafter, the running sum is normalized to derive a numberindicative of the difference between the electrocardiogram beingprocessed and the template. If that number is greater than apredetermined threshold, the heart beat corresponding to theelectrocardiogram being processed is classified as an abnormal heartbeat. Conversely, if that number is less than the predeterminedthreshold, the heart beat corresponding to the electrocardiogram beingprocessed is classified as a normal sinus heart beat.

The foregoing discrimination or classification of the normal heart beatsand the abnormal heart beats is utilized for classifying heart beatrhythms as will be seen hereinafter and for detecting and monitoringischemia. More specifically, the microprocessor 92 processes only thoseelectrocardiograms corresponding to normal sinus heart beats for thepurpose of detecting and monitoring ischemia.

In accordance with the present invention, the template 120 is revised atspaced apart time intervals such as periodically every 15 seconds. After15 seconds has elapsed since the last template revision, themicroprocessor averages the data samples for only thoseelectrocardiograms corresponding to classified normal sinus heart beatsand then computes a weighted average which is then averaged with theprevious template. As a result, revisions to the template willaccurately represent the gradual changes in heart morphology of apatient over time. The incorporation of an adaptive template by therevisions to the template as described above is an important advancementin the art and is considered to be one important element in rendering animplantable cardiac monitor for detecting and monitoring arrhythmias andischemia a reality.

Referring now to FIG. 5, it provides another graphic representation of atypical, normal ECG wave form showing fiducial reference points whichthe processing means 90 determines and utilizes for detecting andmonitoring arrhythmias of the heart. The electrocardiograms of only thefirst and second channels are processed for distinguishing betweennormal sinus heart beats and abnormal sinus heart beats and fordetecting and monitoring arrhythmias. The processing means 90 determinesthree fiducial points for detecting and monitoring arrhythmias. Thethree fiducial points are the start of the QRS complex, the end of theQRS complex, and the peak of the QRS complex which are determined inthat order. When the microprocessor 92 begins processing the datasamples of the electrocardiograms in response to the delayed signal fromthe R wave detector 96, the microprocessor in knowing the delay time ofthe trigger signal can determine approximately when the R wave wasdetected by the R wave detector 96. The microprocessor establishes aninterrogation window which is wide enough to encompass the QRS complex.To determine the three fiducial points, the microprocessor 92 performs aband pass differentiating function upon the stored ECG data whicheliminates the P wave and the T wave from the ECG and performs slopediscrimination of the QRS complex. The start of the QRS complex isdetermined by the beginning of a rapidly increasing slope and the end ofthe QRS complex is determined by the end of a rapidly decreasing slope.After the first two fiducial points are determined, the microprocessor92 then determines when the slope of the QRS complex transitions from arapidly increasing slope to a rapidly decreasing slope. This point oftransition denotes the peak of the QRS complex. By determining thesethree fiducial points, the microprocessor 92 is then able to line up thedata points of the ECG wave forms corresponding to the heart beat beingprocessed with the template as previously described and illustrated inFIG. 4 for comparing the ECG wave forms being processed with thetemplate. Also, from these fiducial points, the microprocessor 92 isable to determine the duration of the QRS complex and for determiningthe R to R interval from the last processed heart beat for determiningheart rates.

Referring now to FIG. 6, it illustrates another graphic representationof a typical or normal ECG wave form showing the fiducial pointsdetermined by the microprocessor 92 for detecting and monitoringischemia. Again, the microprocessor determines three fiducial points,the i point, the j point, and the ST point. In performing thisoperation, the microprocessor processes the digital samples of the ECGwave forms of all four channels, namely, the first, second, third, andfourth channels. For each of the electrocardiograms of these channels,the microprocessor, from the stored digital samples, determines the ipoint as the flat portion known as the isoelectric prior to the firstnegative deflection which is the Q wave before the QRS complex. The jpoint is determined from the first deflection from the S wave and the STpoint is determined to be the point in the electrocardiogram spaced fromthe j point by some predetermined interval, for example, 80milliseconds. The i point is utilized for establishing the baseline forthe ischemia determinations. The ST point is the point in which thedeviation from the baseline at the i point is determined for detectingischemia.

Referring now to FIG. 7, it is an overall flow diagram illustrating themanner in which the implantable cardiac monitor of FIG. 3 may beimplemented for monitoring the physiology of the human heart. Uponinitialization, the microprocessor in step 150 establishes the initialtemplate as previously described, of the type as illustrated in FIG. 4,to be utilized for discriminating between normal sinus heart beats andventricular beats. Once the microprocessor stores the initializedtemplate in the random access memory 84, the microprocessor 92 waits forthe delayed signal from the R wave detector 96 indicating that an R wavehad been detected by the electrodes associated by one or both of thefirst and second channels. As illustrated in FIG. 7, the microprocessor92 receives the delayed signal from the R wave detector 96 in step 152.

After receiving the delayed signal from the R wave detector 96indicating that an R wave had been detected, the microprocessor then instep 154 calculates necessary trigger data. In this step, themicroprocessor determines from the current time when the R wave detector96 must have detected the R wave. The microprocessor is able to discernthe time in which the R wave had been detected by the R wave detector 96because the delay in providing the R wave signal to the microprocessoris a constant delay stored in memory. The trigger point, that is thetime in which the R wave detector 96 detected the R wave, is laterutilized by the microprocessor for establishing time windows in which itanalyzes the ECG data for the heart beat which had been stored in therandom access memory 84 through the direct memory access 88 prior toreceiving the delayed signal from the R wave detector 96.

Upon receipt of the interrupt from interrupt request 98 corresponding tothe delayed signal from the R wave detector 96, the microprocessor thenproceeds to step 156 to analyze the event or heart beat. The executionin analyzing the event or heart beat is denoted by the general referencecharacter 156 and, as will be seen hereinafter, requires a number ofexecutions which are illustrated in the flow diagrams of FIGS. 8 through13 which will be described hereinafter. Generally, in analyzing theevent, the microprocessor 92 performs such functions as determining ifthere was noise in the first and second channels while the ECG data wasstored in the random access memory 84, the microprocessor verifies thatthe first and second channels have ECG data stored therein representinga QRS complex, and the microprocessor determines the fiducial points inthe stored ECG data as previously described with respect to FIG. 5. Themicroprocessor in analyzing the event further classifies the heart beatas a normal sinus heart beat or a ventricular beat by comparing thestored ECG data to the template stored in the random access memory 84.After determining whether the heart beat was a normal sinus heart beator a ventricular beat, the microprocessor then classifies the heartrhythm based upon the currently analyzed heart beat and previous heartbeat history as will be described subsequently.

After analyzing the event or heart beat, the microprocessor then entersa check timer routine at step 158. The check timer routine isillustrated in detail in the flow diagrams of FIGS. 14 through 16 whichwill also be described subsequently. Generally, during the check timerroutine, the microprocessor determines if a time period had elapsedsince last performing the check timer routine. If sufficient time haselapsed, the microprocessor revises the template currently stored in therandom access memory 84, determines the fiducial points illustrated inFIG. 6 for detecting and analyzing ischemia, and then performs theischemia analysis. Once the check timer routine is completed themicroprocessor returns and is deactivated until it receives anotherdelayed R wave detection signal from the R wave detector 96.

Referring now to FIGS. 8A and 8B, these figures when taken together area flow diagram illustrating the manner in which the implantable cardiacmonitor of FIG. 3 may be implemented for analyzing an event or heartbeat as generally illustrated at step 156 in FIG. 7. After determiningthe trigger point, the point in time in which the R wave detector 96detected an R wave, the microprocessor 92 first determines in step 160if it is still in refractory. Even though the microprocessor 92 is notprocessing data during the heart beats of the heart, the crystaloscillator 108 in performing its real time clock function permits themicroprocessor to maintain a refractory counter which is utilized by themicroprocessor to determine if enough time has elapsed since the lastheart beat for the current heart beat to be analyzed to be a valid heartbeat. In other words, if the present trigger point occurred too soonafter the immediately preceding trigger point, the microprocessor 92will know that the R wave detected by the R wave detector 96 cannot be avalid R wave of a QRS complex. The refractory period established by themicroprocessor 92 may be programmable and on the order of 260milliseconds. If the circuitry determines that it is still inrefractory, it then increments the refractory counter in step 162. Afterincrementing the refractory counter, the circuitry determines if therefractory period kept in the refractory counter is now equal to therefractory period of 260 milliseconds. If it is not, signals from the Rwave detector 96 are ignored. However, if the refractory time kept inthe refractory counter after having been incremented is now equal to therefractory period of 260 milliseconds, the microprocessor in step 166sets a bit in the random access memory 84 indicating the end ofrefractory and then returns and terminates processing until the receiptof another delayed R wave detector signal from the R wave detector 96.The bit set in the random access memory 84 will then be utilized thenext time the microprocessor 92 executes step 160 in determining whetherit is still in refractory.

If in performing step 160 the microprocessor 92 determines that it isnot in refractory, it then proceeds to step 168 to detect for noise inthe ECG data stored in the random access memory 84 obtained from thefirst channel. In performing step 168 the microprocessor 92 generatesdata to permit it to determine if there was such noise in the firstchannel. Such data may result from analyzing the ECG data stored in therandom access memory 84 obtained from the first channel for zerocrossings indicated by the data which would not normally occur duringportions of a valid heart beat. For example, the microprocessor 92analyzes the stored data for zero crossings at times which correspond tothe ST segment of the ECG wherein, if the heart beat is a valid heartbeat, the data would indicate a generally constant level. However, ifthere was noise in the first channel, the microprocessor will detectzero crossings resulting from signals of changing directions which wouldnot normally occur during this interval.

After generating the noise data from step 168, the microprocessor thendetermines if there was noise in the first channel when the data wasstored in the random access memory 84. The microprocessor makes thisdetermination at step 170. If it is determined that there was no noisein the first channel, the microprocessor then proceeds to step 172 toperform the noise detection analysis with respect to the data stored inthe random access memory which was obtained from the second channel.However, if there was noise in the first channel, the microprocessorthen proceeds to step 174 to verify that the data stored in the randomaccess memory 84 and obtained from the second channel indicates thatthere was a valid QRS complex in the second channel. Preferably this isaccomplished by discerning if the data stored in the random accessmemory 84 obtained from the second channel was above a given threshold.The verification of a QRS complex in the second channel is performed totake into account the situation where there is noise in the firstchannel and no signal detected in the second channel which indicatesthat the original R wave detection was due to a noise artifact in thefirst channel.

After generating the noise detection data from the second channel instep 172, the microprocessor then in step 176 determines if there wasnoise in the second channel when the data from the second channel wasstored in the random access memory 84. If it is determined that therewas not noise in the second channel, the microprocessor then proceeds tostep 178 to determine if there was a valid heart beat detected. If it isdetermined that there was noise in the second channel, themicroprocessor then performs step 180 to verify the detection of a QRScomplex in the first channel. In performing step 180, the microprocessorperforms the same executions as it did in step 174 but in this case, itperforms these operations upon the data stored in the random accessmemory 84 obtained from the first channel.

In performing step 178 to determine if a valid beat had been detected,the microprocessor utilizes the following criteria. If both the firstand second channels contained noise, the microprocessor will determinethat a reliable beat classification cannot be performed. If themicroprocessor detected that there was noise in one channel and wasunable to verify a detected QRS complex in the other channel, it willdetermine that a valid beat had not been detected. As a result, if instep 178 the microprocessor determines that a valid beat had not beendetected, it will set in step 182 another refractory period of, forexample, 80 milliseconds. This precludes the microprocessor fromprocessing any more data until after this new refractory period haselapsed. This provides sufficient time for any noise in the first andsecond channels to settle down before the microprocessor once againprocesses data period.

Even though the microprocessor may determine in step 178 that there wasa valid heart beat detected, such a determination is consideredconditional by the microprocessor and the microprocessor will make noteof certain characteristics by setting appropriate bits in the randomaccess memory 84 relating to the characteristics detected in the dataobtained from the first and second channel. For example, if themicroprocessor finds that either or both channels included noise eventhough the threshold had been exceeded, it will make note of the noiseon these channels. In addition, the bits which are set denoting thedetected noise will be utilized in analyzing the next event as noisehistory in a manner to be seen hereinafter. The foregoing is bestillustrated in the next step performed by the microprocessor, which isstep 184. In step 184 the microprocessor determines the noise statusbased upon prior noise history. If noise was previously detected in thedata stored for the previous heart beat, and if noise is still present,the microprocessor will proceed to step 182 to set the new refractoryperiod. If noise had not been previously detected with respect to thepreviously detected heart beat, the microprocessor proceeds to step 186to determine the present noise conditions. If there is noise in thestored data of both the first and second channels, the microprocessorthen in step 188 sets the noise status for shutdown. During shutdown,the microprocessor does not process anymore data, as, for example,determining the previously mentioned fiducial points. After shut down,the microprocessor proceeds to step 182 to set the new 80 millisecondrefractory period.

If in step 184 the microprocessor determines that it had previously beenin shutdown but that a valid beat had been detected, then themicroprocessor proceeds to step 190 to determine the fiducial pointsillustrated in FIG. 5 for the data stored in the random access memory 84and obtained from the first and second channels which did not containnoise. The reason for the microprocessor determining these fiducialpoints before exiting is to enable the microprocessor to determine aheart rate for the next detected heart beat. A heart rate could not bedetected for the present beat since the microprocessor had previouslydetected noise in both the first and second channels and therefore couldnot determine the required fiducial points.

In step 186, the microprocessor also determines if both the first andsecond channels did not contain noise or if only one channel containednoise. If one channel contained noise, the microprocessor proceeds tostep 192 to perform a channel analysis only with respect to the dataobtained from the channel having no noise. If both the first and secondchannels contained data without noise, the microprocessor proceeds tostep 194 to perform channel analysis on both the first and secondchannels.

Referring now to FIG. 9, it is an overall flow diagram illustrating themanner in which the implantable cardiac monitor of FIG. 3 may beimplemented for performing the channel analysis of the flow diagram ofFIGS. 8A and 8B in accordance with the preferred embodiment. Aspreviously mentioned, the microprocessor 92 performs the channelanalysis upon the data stored in the random access memory 84 obtainedfrom either or both of the first and second channels depending upon thedetected noise conditions of the stored data. As a result, the channelanalysis may be performed on either the first channel data or the secondchannel data or upon both the data obtained from the first and secondchannels.

The channel analysis basically includes the steps 200 and 202 wherein,in step 200, the microprocessor determines the fiducial pointsillustrated in FIG. 5 and, where, in step 202, the microprocessorclassifies the detected heart beat. Step 200 to determine the fiducialpoints will be described hereinafter with respect to FIG. 10 and step202 will be described hereinafter with respect to the flow diagrams ofFIGS. 11 through 13.

Referring now to FIG. 10, it is a flow diagram illustrating the mannerin which the implantable cardiac monitor of FIG. 3 may be implementedfor determining the fiducial points illustrated in FIG. 5 in accordancewith this preferred embodiment. As previously mentioned, and asillustrated in FIG. 5, the microprocessor determines three fiducialpoints for arrhythmia analysis, the three fiducial points being theleading edge of the QRS complex, the trailing edge of the QRS complex,and the peak of the QRS complex.

To locate the three fiducial points, the microprocessor establishes afiducial window wherein it processes the ECG data stored a predeterminedtime before to a predetermined time after the trigger point. Inaccordance with this preferred embodiment, the microprocessorestablishes a fiducial window by processing ECG data stored 160milliseconds before to 100 milliseconds after the trigger point.

The microprocessor, in step 210, first locates the first fiducial pointof the QRS complex leading edge. As previously described, themicroprocessor accomplishes this by differentiation to find the firstmajor slope preceding the Q wave. Next, in step 212, the microprocessordetermines the second fiducial point which is the trailing edge of theQRS complex by differentiation to find the last major slope of the QRScomplex. Next, in step 214, the microprocessor 92 identifies the maximumslope points as the first and second fiducial points. Thereafter, instep 216, the microprocessor determines the polarity of the filteredsignal of the template. If the template polarity is monophasic, themicroprocessor in step 218 sets the template start point as the maximumslope. If however the template polarity is biphasic, the microprocessorin step 220 sets the template start point to the second peak slope.

Following step 220, the microprocessor determines the peak of the QRScomplex by first, in step 222, searching for the zero crossing point ofthe filtered signal of the template. Next, in step 224, themicroprocessor processes the ECG data for noting the zero crossing pointas the third fiducial point which is the peak of the QRS complex.Lastly, in step 226, the microprocessor computes the duration of the QRScomplex by subtracting the time of the leading edge from the time of thetrailing edge of the QRS complex.

After determining the fiducial points as described with respect to FIG.10, the microprocessor then classifies the heart beat as illustrated inthe flow diagrams of FIGS. 11 through 13. Referring more particularly toFIG. 11, it is a flow diagram illustrating the manner in which theimplantable cardiac monitor of FIG. 3 may be implemented for classifyingheart beats in accordance with this preferred embodiment of the presentinvention. The microprocessor begins at step 230 to classify the heartbeats by determining whether it is to perform the beat classification onthe data obtained from both the first and second channels or upon thedata obtained from only one of the channels. If the microprocessordetermines that it is to classify the heart beat on the data obtainedfrom just one of the channels, it will proceed to step 232 to performthe template match to be described hereinafter with respect to FIG. 12on the data obtained from the one channel. Such one channel analysiswill occur when one channel has detected a valid QRS complex and, forexample, when the other channel had noise when providing the randomaccess memory 84 with ECG data.

If the microprocessor is to perform channel analysis with respect to theECG data obtained from both the first and second channels, it will firstexecute a template match in step 234, to be described hereinafter withrespect to FIG. 12, upon the data obtained from the first channel. Afterperforming the template match in step 234 upon the data obtained fromthe first channel, the microprocessor determines whether the heart beatwas a normal sinus heart beat or a ventricular beat. If the heart beatwas a ventricular beat, the microprocessor jumps to step 242 to recordthe heart beat as a ventricular beat. However, if in step 236 themicroprocessor determines that the heart beat was a normal sinus heartbeat and not a ventricular beat, it will proceed to step 238 to performthe template match upon the data obtained from the second channel.Thereafter, in step 240, the microprocessor determines whether the heartbeat was a normal sinus heart beat or a ventricular beat based upon thestored data obtained from the second channel. If the microprocessor hadperformed a single channel analysis in step 232, it would then go tostep 240 to determine, based upon the template match, if the heart beathad been a normal sinus heart beat or a ventricular beat. In eithercase, if the microprocessor determines in step 240 that the heart beatwas a ventricular beat, it would proceed to step 242 to record the heartbeat as a ventricular beat. However, if in step 240, the microprocessordetermines that the heart beat was a normal sinus heart beat, it wouldrecord it in step 244 as a normal sinus heart beat.

After recording the heart beat as a ventricular beat or a normal sinusheart beat, the microprocessor then proceeds to step 246 to calculatethe heart rate based upon this last heart beat and a running averageheart rate based upon the last predetermined number of normal sinus R toR intervals, such as the last six normal sinus R to R intervals. Themicroprocessor stores in the random access memory 84 both the heart ratebased upon the last heart beat and the running average heart beat. Aftercompleting step 246, the microprocessor then proceeds to step 248 forclassifying the heart rhythm.

Referring now to FIG. 12, it is a flow diagram illustrating the mannerin which the implantable cardiac monitor of FIG. 3 may be implementedfor comparing the ECG data stored in the random access memory 84 andobtained from the first channel, the second channel, or both the firstand second channels to perform the template match operations tofacilitate the classification of the heart beat. In performing thetemplate match operation, the microprocessor first, in step 250, alignsthe ECG data with the stored template. In performing this step, themicroprocessor aligns the maximum point of the QRS complex with thestored template. Thereafter, in step 252, the microprocessor establishesan analysis window beginning a predetermined time before the QRS complexand ending at a predetermined time following the QRS complex. Inaccordance with this preferred embodiment, the analysis window beginseight milliseconds before the QRS complex and extends to eightmilliseconds after the QRS complex of the stored ECG data. Hence, inaccordance with the present invention, in performing the template matchoperations, the microprocessor compares the QRS complex portion of thestored ECG data with the QRS complex of the stored template.

After establishing the analysis window in step 252, the microprocessorproceeds to step 254 to calculate the point to point differences betweenthe data points of the stored ECG data and the stored ECG template. Ifthe point to point difference exceeds the maximum or minimum limits ofthe stored ECG template, the microprocessor sums the differences in therandom access memory 84.

Following the calculations of the point to point differences between thestored ECG data and the stored ECG template, the microprocessor then instep 260 normalizes the total sum of the differences by dividing thetotal sum of the differences by some percentage, such as, for example,twenty-five percent (25%) of the integral of the QRS complex of thetemplate to determine a normalized correlation value. If in step 256 itis determined that there are no data points outside of the template, themicroprocessor does not calculate the sum of the differences but insteadproceeds directly to step 260 for determining the normalized correlationvalue.

Following the template match operations illustrated in FIG. 12, themicroprocessor then, by utilizing the normalized correlation value,classifies the heart beat as either a normal sinus heart beat or anabnormal heart beat herein referred to as a ventricular beat asdescribed previously with respect to FIG. 11. The microprocessor thenrecords the classification of the heart beat in the random access memory84.

Referring now to FIGS. 13A and 13B, these figures when taken togetherform a flow diagram illustrating the manner in which the implantablecardiac monitor of FIG. 3 may be implemented for classifying heart beatrhythms in accordance with this preferred embodiment of the presentinvention. The heart beat rhythms are classified after a heart beat hasbeen classified, after the R to R interval with respect to theimmediately preceding R wave has been determined, and after the heartrate corresponding to the presently analyzed heart beat and the runningheart rate average have been determined.

The microprocessor begins the rhythm classification by determining instep 260 if the heart beat had been classified as a ventricular beat. Ifthe heart beat had been classified as a ventricular beat, themicroprocessor proceeds to step 262 to increment the total ventricularbeat counter in the random access memory 84. Next, in step 264, themicroprocessor increments the ventricular beat string counter formaintaining a total of the number of consecutive ventricular beats whichhave been detected. Thereafter, the microprocessor proceeds to the checktimer operations to be described hereinafter.

If in step 260 the microprocessor found in the random access memory 84that the heart beat was not a ventricular beat, it proceeds to step 266to determine if the previous heart beat had been classified as aventricular beat. If the previous heart beat had been a ventricularbeat, the microprocessor then proceeds to step 268 to determine if thecount in the ventricular beat string counter is equal to one. If thecount in the ventricular string counter is not equal to one, themicroprocessor proceeds to step 270 to determine if the count in theventricular string counter is equal to two. If the ventricular stringcount is equal to two, the microprocessor then in step 272 records acouplet in the random access memory 84. Such a couplet is the occurrenceof two consecutive ventricular beats.

If in step 270 the microprocessor determines that the ventricular beatstring count is not equal to two, it then proceeds to step 274 todetermine if the count in the ventricular beat string counter is equalto three. If the ventricular beat string count is equal to three, themicroprocessor then in step 276 records a triplet in the random accessmemory 84. Such a triplet is the occurrence of three consecutiveventricular beats. If the count in the ventricular string counter is notequal to three, the microprocessor then proceeds to step 278 todetermine if the count in the ventricular string counter is greater thana predetermined threshold count. If the count in the ventricular beatstring counter is greater than three but less than the predeterminedthreshold count, the microprocessor then in step 280 records in theevent counter of the random access memory 84 a non-sustained ventriculartachycardia, clears the ventricular beat string counter in step 282, andthen proceeds to the check timer operations to be described hereinafter.As can thus be seen, a plurality of heart beats forms a heart rhythmand, therefore, the microprocessor stores in the random access memory 84the classification of the previous heart beats for classifying heartbeat rhythms. In addition, the implantable cardiac monitor classifiesthe heart beat rhythms only after detecting and classifying a heart beatas a normal sinus heart beat.

If in step 278 the microprocessor determines that the ventricular beatstring count is greater than the predetermined threshold count, themicroprocessor then proceeds to step 284 to record the heart beat rhythmas a ventricular tachycardia in the arrhythmia log of the random accessmemory 84. In recording the ventricular tachycardia in the arrhythmialog of the random access memory 84, the microprocessor time stamps therecorded ventricular tachycardia as to the date and time in which it wasrecorded. This enables the cardiologist, upon retrieving this data fromthe implanted cardiac monitor to be informed as to when the ventriculartachycardia occurred. In addition, in step 284, the microprocessor movesthe ECG data corresponding to the electrocardiograms generated duringthe ventricular tachycardia to the ECG storage portion of the randomaccess memory 84 to facilitate retrieval of the storedelectrocardiograms by the cardiologist. In accordance with thispreferred embodiment, the microprocessor stores the first and lastelectrocardiograms generated during the ventricular tachycardia forretrieval by the cardiologist.

After completing step 284, the microprocessor then in step 282 clearsthe ventricular beat string counter. It then proceeds to perform thecheck timer operations to be described hereinafter.

Returning now to step 268, if the microprocessor determines that theventricular beat string count is equal to one, it then proceeds to step288 which increments the bigeminy string counter of the random accessmemory 84. As well known in the art, a bigeminy rhythm is a heart beatrhythm having cycles of consecutively alternating normal sinus heartbeats and ventricular beats. After incrementing the bigeminy stringcounter, the microprocessor then performs step 282 to clear theventricular beat string counter. If there has been more than one suchcycle of alternating normal and ventricular heart beats, themicroprocessor proceeds from step 288 to step 290 to determine if therehas been a bigeminy rhythm. If there has been a bigeminy rhythm, themicroprocessor in step 292 records a bigeminy rhythm in the bigeminyevent counter of the random access memory 84 and then increments thebigeminy string counter in accordance with step 288. Thereafter, themicroprocessor clears the ventricular beat string counter in step 282and ends.

If in step 290 it was determined that there was not a bigeminy rhythm,the microprocessor then in step 294 determines if the previousventricular beat had been premature. In making this determination, themicroprocessor retrieves the R to R interval stored in the random accessmemory 84 corresponding to the previous ventricular beat and compares itto a threshold which may be programmed by the cardiologist. If theprevious ventricular beat was premature, the microprocessor then in step296 records a premature ventricular contraction (PVC) in the PVC eventcounter. Thereafter, the microprocessor performs step 282 and ends.

If in step 294 it was determined that the previous ventricular beat wasnot premature, the microprocessor then proceeds to step 298 to record aventricular ectopic beat in the ventricular ectopic beat event counter.It then proceeds to perform step 282 and ends.

Returning now to step 266, if in step 266 the microprocessor determinesthat the previous heart beat had not been a ventricular beat afterhaving determined that the present heart beat is a normal sinus heartbeat, the microprocessor then proceeds to step 300 to determine if therehas been a bigeminy rhythm. If there has been a bigeminy rhythm, themicroprocessor then proceeds to step 302 to determine whether there hasbeen a sustained bigeminy rhythm. The microprocessor performs step 302by determining if the bigeminy string counter contains a count which isgreater than a predetermined number of counts. If the microprocessordetermines in step 302 that there has been a sustained bigeminy rhythmit records and time stamps the bigeminy rhythm event in the arrhythmialog. The microprocessor also records the bigeminy string counter countin the arrhythmia log in step 304. Also in step 304 the microprocessorstores relevant data such as rhythm duration generated during thebigeminy rhythm in the arrhythmia log of the random access memory 84 forretrieval by the cardiologist. To that end, the microprocessor isresponsive to the first incrementing of the bigeminy string counter formaintaining the data in the random access memory so that such data isavailable for transmission by the telemetry means to the cardiologistshould a sustained bigeminy rhythm be determined.

Following step 304 and if in step 302 the microprocessor determines thatthere has not been a sustained bigeminy rhythm, the microprocessorproceeds to step 306 for clearing all bigeminy counters. Thereafter, themicroprocessor ends and proceeds to the check timer operations to bedescribed hereinafter.

If in step 300 the microprocessor determines that there has not been abigeminy rhythm, the microprocessor then proceeds to step 308 todetermine if the present normal sinus heart beat was premature. Inperforming step 308, the microprocessor compares the R to R intervalcorresponding to the present normal sinus heart beat to a predeterminedthreshold. If the R to R interval is less than the predeterminedthreshold, the microprocessor in step 310 records the present heart beatas a premature atrial contraction (PAC) in the PAC event counter of therandom access memory 84. Thereafter, the microprocessor ends and entersthe check timer operations.

If in step 308 the microprocessor finds that the present beat was notpremature, it then proceeds to step 312 to determine if there has been ahigh sinus rate. In performing step 312, the microprocessor compares thedetermined average heart rate to a predetermined heart rate which may beprogrammed by the cardiologist. If the microprocessor determines in step312 that there has not been a high sinus rate, it ends. However, if itdetermines in step 312 that there has been a high sinus rate, it thenproceeds to step 314 to check for an irregular heart rate. Then, in step316, it determines if there has been an irregular heart rate. If therehas not, the microprocessor ends. However, if there has been anirregular heart rate, the microprocessor then proceeds to step 318 todetermine if there has been a sustained irregular heart rate. If therehas been a sustained irregular heart rate, the microprocessor then instep 320 records and time stamps the irregular rhythm in the arrhythmialog, saves an ECG strip in the random access memory 84, and then ends.If there has not been a sustained irregular heart rhythm, themicroprocessor then in step 322 records an irregular rhythm in theirregular event counter of the random access memory 84. After each ofsteps 320 and 322, the microprocessor ends and proceeds to the checktimer operations to be described subsequently.

Referring now to FIG. 14, it is an overall flow diagram illustrating themanner in which the implantable cardiac monitor of FIG. 3 may beimplemented for revising the ECG template and performing ischemiaanalysis at spaced apart time intervals. The microprocessor begins atstep 330 by updating the real time clock 108 illustrated in the blockdiagram of FIG. 3. After updating the real time clock in step 330, themicroprocessor proceeds to step 332 for determining if the spaced apartor averaging interval has been completed. As previously mentioned, thisinterval may be fifteen seconds. If the interval has not completed, themicroprocessor ends to complete its processing of data for this heartbeat and is deactivated until it receives another delayed R wave detectsignal from the R wave detector 96. However, if the averaging timeinterval has completed, the microprocessor proceeds to step 334 toanalyze the ECG template and determine ischemia analysis fiducial pointsas will be described subsequently with respect to FIG. 15. Themicroprocessor then proceeds to step 336 for performing the ischemiaanalysis which will be described hereinafter with respect to FIG. 16.

Referring now to FIG. 15, it is a flow diagram illustrating the mannerin which the implantable cardiac monitor of FIG. 3 may be implementedfor revising the ECG template at the spaced apart time intervals anddetermining ischemia analysis fiducial points in accordance with thispreferred embodiment of the present invention. The microprocessor beginsat step 340 by calculating the average of the ECG data generated sincethe last template revision for those heart beats corresponding to normalsinus heart beats. Hence, the microprocessor in step 340 averages theECG data for selected ones of the electrocardiograms generatedresponsive to the heart beats which occurred since the last templaterevision. In step 342 the microprocessor calculates filtered data of theaveraged heart beat calculated in 340 and then in step 344 updates thethreshold levels of the averaged heart beat. Then, in step 346, themicroprocessor determines the new template wave shape and sets the newmaximum and minimum levels for the revised template. In performing step346, the microprocessor relies upon the previous template and revisesthe previous template in accordance with the weighted average of theaveraged heart beat calculated in step 340.

After the revised template has been determined, the microprocessorlocates the i and j fiducial points of the ECG corresponding to thepresent heart beat in step 348. Such i and j points have been previouslyshown and described with respect to FIG. 6. Next, in step 350, themicroprocessor measures the template baseline, that is, the level of thei point of the revised template. Then, in step 352, the microprocessordetermines the deviation in the ST level between the template baselineand the level at some predetermined point, for example, 80 millisecondsafter the j point, to determine ST level deviation at the ST point.Thereafter, the microprocessor ends and enters the ischemia analysisillustrated in FIG. 16.

Referring now to FIG. 16, it is a flow diagram illustrating the mannerin which the implantable cardiac monitor of FIG. 3 may be implementedfor performing ischemia analysis in accordance with this preferredembodiment of the present invention.

The microprocessor begins at step 360 to determine if the magnitude ofthe ST level deviation is greater than a threshold level of either STelevation or ST depression. Such a threshold level is preferablyprogrammable by the cardiologist. If the ST level deviation is greaterthan the threshold, the microprocessor proceeds to step 362 to incrementthe ischemia counter of the random access memory 84. The microprocessorthen, in step 364, determines if the duration of the ischemia is greaterthan an episode threshold. If it is not, the microprocessor ends. Ifhowever the duration of the ischemia is greater than an episodethreshold, the microprocessor then in step 366 updates the ischemiaepisode data in the random access memory 84. Such data may include thecurrent duration of the ischemic episode. The microprocessor then ends.

In step 360 if the microprocessor finds that the ST level deviation isless than the threshold, the microprocessor then proceeds to step 362 todetermine if this is an end of an ischemic episode. In performing step362, the microprocessor determines if ischemia data has been retained inthe random access memory. If the microprocessor fails to detect ischemiadata retained in the random access memory, it will then end. However, ifthe microprocessor finds ischemia data in the random access memory inperforming step 362, it proceeds to step 368 to record an ischemicepisode in the ischemia event counter of the random access memory 84.Hence, as can be seen, the implantable cardiac monitor characterizesischemia episodes after there has been detected ischemia immediatelyfollowed by the determination of an ST level deviation which is lessthan the threshold limit.

After step 368, the microprocessor proceeds to step 370 to add theduration of this last detected ischemic episode to the ischemia trenddata stored in the random access memory 84. Next, in step 372, themicroprocessor determines if the ischemic episode was a sustainedepisode. If it was not sustained, the microprocessor ends. If theischemic episode was sustained, that is, if the ischemia episodeduration was greater than a predetermined time, the microprocessor thenproceeds to step 374 to record the ischemic episode in the ischemicepisode log of the random access memory 84. Also, at this time, themicroprocessor transfers the ECG data generated during the sustainedischemia episode to the ECG strip store of the random access memory 84for retrieval by the cardiologist through the telemetry. Such ischemiadata is maintained within the random access memory by the microprocessorupon the detecting of an onset of ischemia in accordance with step 360.Also, when the sustained ischemic episode is recorded in the log, thesustained ischemic episode is also time stamped by the microprocessor.

Referring now to FIG. 17, it is a front plan view of an implantablecardiac monitor 30 including an electrode system employing catheterelectrodes for sensing heart activity and configured in accordance withone preferred embodiment of the present invention. The implantablecardiac monitor 30 includes an enclosure 32 for containing electricalcircuitry, preferably as previously described herein, for monitoring thephysiology of the human heart. The enclosure includes an upper perimeter400. The electrode system includes an electrically insulating header 402sealingly engaged with the upper perimeter 400 of the enclosure 32,first and second flexible insulative conduits 404 and 406 respectivelyextending from the header 402, and first and second electrode means 34and 36 carried by each of the first and second conduits 404 and 406respectively. The first and second electrode means 34 and 36 eachinclude three spaced apart electrically conductive electrodes with thefirst electrode means including electrodes 408, 410, and 412, and thesecond electrode means 36 including electrodes 414, 416, and 418.Conductor means 420 extends through the first conduit 404 and into theheader 402 for coupling each of the electrodes 408, 410, and 412individually to the electrical circuitry within the enclosure 32.Similarly, conductor means 422 extends through the second conduit 406and into the header 402 for individually coupling the electrodes 414,416, and 418 to the electrical circuitry within enclosure 32.

In accordance with this preferred embodiment, each of the electrodes408, 410, 412, 414, 416, and 418 is ring-shaped and has an outerdiameter which is substantially equal to the outer diameter of theflexible insulative conduits 404 and 406. As a result, when the cardiacmonitor 30 is implanted beneath the skin of a patient, each of theelectrodes will make electrical contact with the heart for detectingheart activity. Preferably, the electrodes upon implantation are placedbeneath the skin of a patient to dispose the electrodes in nontouchingproximity to the heart for establishing the electrical contact betweenthe electrodes and the heart. Also, it will be noted that the first andsecond conduits 404 and 406 extend from the header 402 in asubstantially V-shaped configuration so that, when the enclosure 32 isimplanted as illustrated in FIG. 1, the electrodes will be in closeproximity to the V₂ to V₆ precordial locations as illustrated in FIG. 1.

Each of the first and second conduits 404 and 406 also include a suturemeans 424 and 426 respectively which are integrally formed in theflexible conduits and include a pair of holes 428 and 430 in suturemeans 424 and 432 and 434 in suture 426. With this configuration, thesuture means permit the first and second conduits 404 and 406 to besutured in place for providing fixation for the electrodes of the firstand second electrode means 34 and 36.

Referring now to FIG. 18, it is a front plan view of another implantablecardiac monitor 30 employing catheter electrodes for sensing heartactivity and is configured in accordance with another preferredembodiment of the present invention. Again, the cardiac monitor includesan enclosure 32, an insulative header 436 which includes a firstconnector receptacle 438 and a second connector receptacle 440.

First and second insulative conduits 444 and 446 each carry a pair ofspaced apart ring-shaped catheter electrodes with conduit 444 carryingelectrodes 448 and 450 and conduit 446 carrying electrodes 452 and 454.Conductor means 456 extends through conduit 444 to individuallyelectrically couple electrodes 448 and 450 to the contacts of anelectrical connector 460. Similarly, conductor means 458 extends throughconduit 446 for individually electrically connecting electrodes 452 and454 to the contacts of a second connector 462. This permits theelectrodes 448, 450, 452, and 454 to be electrically coupled to theelectrical circuitry within enclosure 32.

The enclosure 32 includes a surface portion 464 which includes anelectrically conductive portion 466. Upon implantation of the cardiacmonitor 30, the conductive portion 466 provides a ground reference forthe sensing of heart activity by the electrodes 448, 450, 452, and 454.Also, it will be noted in the figure, that the conduits 444 and 446 areresiliently preformed so as to extend from the header 436 in asubstantially V-shaped configuration for reasons previously described.

In forming the electrically conductive portion 466 on surface 464 ofenclosure 32, the enclosure 32 may be formed of insulative material andthe conductive portion 466 formed by a coating of electricallyconductive material. Alternatively, the enclosure may be formed fromelectrically conductive material which is covered by an insulativecoating except for the portion 466 to expose the conductive portion 466beneath insulative coating.

Referring now to FIG. 19, it illustrates another preferred embodiment ofa cardiac monitor 30 embodying the present invention. The embodiment ofFIG. 19 is similar to the embodiment of FIG. 18 in that the enclosure 32includes an insulative header 470 which includes first and secondconnector receptacles 472 and 474. The flexible conduits 476 and 478each carry three spaced apart ring-shaped catheter electrodes withconduit 476 carrying electrodes 480, 482, and 484, and conduit 478carrying electrodes 486, 488, and 490. First conductor means 492electrically couples the electrodes 480, 482, and 484 to the contacts ofa first connector 494 which is arranged to mate with the connectorreceptacle 472. Similarly, second conductor means 496 electricallycouples the electrodes 486, 488, and 490 to contacts of a secondconnector 498 which is arranged to mate with the connector receptacle474. As a result, with the contacts of the receptacles 472 and 474 beingcoupled to the electrical circuitry contained within the enclosure 32,the electrodes are coupled to the electrical circuitry.

As in the embodiment of FIG. 18, the conduits 476 and 478 areresiliently preformed so that they extend in a substantial V-shapedconfiguration from the header 470. Like the embodiment of FIG. 17, theconduits 476 and 478 each include suture means 500 and 502 integrallyformed therein for fixing the electrodes in place upon implantation ofthe cardiac monitor 30. As may also be noted in FIGS. 18 and 19, theconnector receptacles 460 and 462 and 472 and 474 are disposed inopposing relation. This permits the conduits to directly extend from theheaders in opposed relation.

Referring now to FIG. 20, it illustrates another implantable cardiacmonitor 30 embodying further aspects of the present invention. As willbe seen hereinafter, the cardiac monitor 30 may be utilized in aleadless configuration wherein the electrical conduits and electrodescarried thereby are not utilized or a configuration where the conduitsand electrodes carried on the conduits are utilized. The cardiac monitorillustrated in FIG. 20, as in the previous embodiments of FIGS. 17-19includes an enclosure 32 for enclosing the electrical circuitry of thecardiac monitor. An insulative header 504 sealingly engages an upperperimeter 506 of the enclosure 32. The header includes a first connectorreceptacle 508 and a second connector receptacle 510 disposed in opposedrelation. The receptacles 508 and 510 are arranged for matinglyreceiving first and second connectors 512 and 514 respectively. Theconnectors 508 and 510 are coupled to conduits 516 and 518 respectively.Each of the conduits 516 and 518 carry an electrode means 34 and 36respectively. The electrode means 34 and 36 comprise strip electrodeswhich include discrete, pill-shaped, conductive electrodes withelectrode means 34 including a pair of electrodes 520 and 522 andelectrode means 36 including electrodes 524 and 526. The strip electrodemeans 34 and 36 each include an elongated strip of flexible material 528and 530 respectively with the discrete electrodes 520 and 522 and 524and 526 embedded therein but having an exposed major surface forestablishing electrical contact with the heart for sensing heartactivity. Conductors 532 and 534 extend through the flexible strip 528and conduit 516 for coupling electrodes 520 and 522 to the contacts ofconnector 512. Similarly, conductors 536 and 538 extend through flexiblestrip 530 and conduit 518 for electrically coupling electrodes 524 and526 to the contacts of connector 514. With the connectors being receivedby the receptacles, and the contacts of the receptacles being coupled tothe electrical circuitry within enclosure 32, the electrodes are coupledto the electrical circuitry to enable the cardiac monitor to monitor thephysiology of the heart.

As will also be noted, the conduits 516 and 518 each include a suturemeans 540 and 542 of the type previously described. As a result, whenthe cardiac monitor 30 is implanted, the suture means 540 and 542 permitthe electrodes to be fixed in place.

In addition to the foregoing, the header 504 also includes conductiveportions 544, 546, and 548. Also, the enclosure 32 includes a conductiveportion 550. When the cardiac monitor 30 is to be utilized for detectingand analyzing arrhythmias, it may be unnecessary to utilize theelectrode means 34 and 36. Instead, the conductive portions 544 and 546may be utilized for monitoring heart activity with conductive portion550 serving as a ground reference for such sensing. Conductive portion548 may also be utilized for sensing or, alternatively, may be utilizedfor providing a low energy subcutaneous stimulus to the patient for thepurpose of providing alarms to the patient as previously described.

In addition to the suture means 540 and 542 provided on conduits 516 and518, for fixing the electrode means 34 and 36 in place uponimplantation, it will be noted that the elongated strips 528 and 530each include a pair of longitudinal side walls 552 and 554 and 556 and558. Extending from the longitudinal side walls 552, 554, 556, and 558are fixation projections 560. The projections 560 form acute angles withthe longitudinal side walls 552, 554, 556, and 560. Upon implantation ofthe cardiac monitor 30, tissue will grow around the fixation projections560 for fixing the electrode means 34 and 36 in place to assurestability of the implantable cardiac monitoring system.

Referring now to FIG. 21, it illustrates another implantable cardiacmonitor 30 embodying the present invention. The cardiac monitorillustrated in FIG. 21 is similar to the cardiac monitor illustrated inFIG. 20 and therefore the similarities need not be described in detailherein. However, it is to be noted that the enclosure 32 and the header562 of the implantable cardiac monitor 30 do not include the conductivesurface portions as illustrated in FIG. 20. In addition, the electrodemeans 34 and 36 each include three discrete, pill-shaped, conductiveelectrodes with electrode means 34 including electrodes 564, 566, and568, and electrode means 36 including electrodes 570, 572, and 574.

Referring now to FIG. 22, it illustrates another implantable cardiacmonitor 30 embodying the present invention. The cardiac monitor 30 ofFIG. 22 is essentially identical to the cardiac monitor illustrated inFIG. 20 except that the conduits 516 and 518 are of a length to enablethe conduit means to be looped around the enclosure 32 to provide strainrelief for the conduit means 516 and 518 between the suture means 540and 542 and the header 504. This gives the cardiologist a greater degreeof flexibility in locating the position of the enclosure 32 uponimplantation of the implantable cardiac monitor of FIG. 22.

Referring now to FIG. 23, it illustrates another implantable cardiacmonitor 30 which is similar to the implantable cardiac monitor of FIG.22. Here, it will be seen, that the connector receptacles 576 and 578are arranged within the header 580 in non-opposing relation. Like theprevious embodiment however, the conduits 582 and 584 are of sufficientlength to enable the conduit means 582 and 586 to be looped around theenclosure 32 to provide strain relief for the conduits 582 and 584between the suture means 586 and 588.

Referring now to FIG. 24, it is a top plan view of one of the stripelectrodes in conjunction with a positioning tool which may be utilizedin accordance with the present invention for positioning the stripelectrode during the implantation thereof. FIG. 25 is a side plan viewof the strip electrode of FIG. 24. For purposes of this discussion, itwill be assumed that the strip electrode illustrated in FIGS. 24 and 25is strip electrode 34 of FIG. 20. As can be seen in FIGS. 24 and 25, theelongated strip of flexible material 528 includes a slot 590 within thetop surface 592. An elongated tool 594 has a distal end 596 configuredto be received within the slot 590. This facilitates the movement of thestrip electrode 34 into a desired position upon implantation of thestrip electrode beneath the skin of a patient. To facilitate suchpositioning, the tool 594 includes a handle 598 at the proximal endthereof which may be gripped by the cardiologist.

Referring now to FIG. 26, it is a front plan view of another implantablecardiac monitor 30 embodying the present invention which employs aleadless electrode system. The cardiac monitor 30 of FIG. 26 includes ahermetically sealed enclosure 32 defining a cavity having an openedperimeter 600 and a header 602 sealingly engaging the perimeter 600. Thecardiac monitor 30 further includes first and second electricalconductors 604 and 606 respectively which cover first and secondrespective discrete portions of the enclosure for forming first andsecond sensing electrodes respectively for sensing activity of theheart. In accordance with this preferred embodiment, the first andsecond electrical conductors cover respective portions of the header602.

The cardiac monitor 30 further includes a third electrical conductor 608covering a third discrete portion of the enclosure for forming areference electrode. The circuit means utilized within the enclosure 32may be the electrical circuitry previously described in accordance withthe preferred embodiment of the present invention.

It will be noted that the first and second electrical conductors 604 and606 are equally spaced from the third electrical conductor 608. Thisensures symmetrical sensing of heart activity at the electrodes 604 and606. In forming the electrodes 604 and 606, the header may be formedfrom electrically insulating material with the first and secondelectrical conductors being an electrically conductive coding coveringthe respective discrete portion of the header.

Referring now to FIG. 27, it illustrates a still further implantablecardiac monitor 30 embodying the present invention. The cardiac monitor30 of FIG. 27 is similar to the cardiac monitor 30 of FIG. 26 exceptthat in addition to electrodes 604 and 606, it additionally provides forelectrode means 34 and 36 of the type previously described. However, aswill be noted in FIG. 27, the cardiac monitor 30 there illustrated alsoincludes a second header 610 in addition to the header 602. Theenclosure 32 includes an additional perimeter 612 with the additionalheader 610 sealingly engaging the additional perimeter 612. Within theadditional or second header 612 is the telemetry means coil antenna 116as illustrated in FIG. 3. Because the telemetry means is located withinthe insulative header 610, efficient telemetry of data to be retrievedby the cardiologist will be provided. In accordance with this preferredembodiment, the enclosure 32 may enclose the electrical circuitrypreviously described.

In addition to the foregoing, the cardiac monitor of FIG. 27 includes afurther discrete electrode 614. This further electrode 614 may beutilized for providing the aforementioned low energy subcutaneousstimulations to the patient for purposes of providing the patient withthe alarms as previously described.

While particular embodiments of the present invention has been shown anddescribed, modifications may be made, and it is therefore intended inthe appended claims to cover all such changes and modifications whichfall within the true spirit and scope of the invention.

What is claimed is:
 1. An electrode system for use with a fullyimplantable cardiac monitor of the type including electrical circuitryfor monitoring the physiology of the human heart and having an enclosurefor containing said electrical circuitry, said electrode systemcomprising:first and second flexible insulative conduits; first andsecond electrode means carried by each of said first and second conduitsrespectively, said first and second electrode means comprising stripelectrode means including an elongated strip of flexible materialincluding a plurality of spaced apart discrete conductive electrodesembedded therein, said electrodes having an exposed major surface, eachsaid elongated strip of flexible material including opposinglongitudinal walls and a plurality of spaced apart fixation projectionsprojecting from said longitudinal walls, said fixation projectionsforming acute angles with said longitudinal walls; conductor meansextending through said first and second conduits and into said headerfor coupling said electrodes of said first and second electrode means tosaid electrical circuitry, said conduits and said electrode means beingimplantable beneath the skin of a patient to dispose said electrodes innon-touching proximity to the heart for establishing electrical contactbetween said electrodes and the heart.